Stoker control system



Dec. 9, 1941. v. R. TATE STOKER CONTROL SYSTEM Filed Nov. 30, 1939 INVENTOR periods of time.

ltuyres. I

' It is one of the important objects of the inv Patented Dec. 9 1941UNITED STATES PATENT OFFlCE STOKER CONTROL SYSTEM Virgil Robins Tate,Milwaukee, Wis, assignor to Perfex Corporation,

Milwaukee, Wis, a corporation of Wisconsin Application November 2Claims.

heated so as to maintain the space at a comfortable temperature.

It is also customary to provide an interval timer for periodicallyoperating the motor independent of the room thermostat in order toprevent the fire from becoming extinguished during long stand-byintervals such as may be encountered in mild weather when the thermostatdoes not call for heat over long In the operation of such a system it isdiflicult to maintain a substantially uniform thickness of fuelbedconsistent with the demands of heat by the system. This is due-toeventually becomes extinguished; or alternately the fuel bed becomes toothin resulting in the formation of blow-holes in the fire bed throughwhich the combustion air rushes unrestrained and the fire graduallyburns itself out. These difiiculties are further aggravated when thecharacter of the fuel varies, when caking occurs, or the formation ofslag is such as to close the retort vention to provide a control systemfor a stoker in which the fuel and combustion air fed to the fire aresubstantially automatically proportioned in accordance with the demandfor heat by the system. v

A further object is to provide a stoker control system which is adaptedto continuously maintain the fuel bed upon the retortat substantially apredetermined thickness in order to render the stoker operationeillcient at all times'regardless of the heating requirements under allconditions.

A further object is to provide an arrangement for a stoker in which thefuel feeding means'is operated and controlled independent of thecombustion air supply means, thereby enabling these Y devices to moreefficiently perform the-required Arranged upon the air duct II andconnected 30, 1939, Serial No. 306,806

function of maintaining a constant depth of fuel on the fire bed.

Another object of the invention is to provide a control system in whichthe fuel feeding motor is controlled in accordance with predeterminedstatic pressure conditions existing in the air duct through which thecombustion air is fed to the stoker retort. This predetermined staticpressure is predicated upon the fuel bed reaching a particular depth inthe retort.

A further object is to provide a system em bodying a circuit arrangementin which the high voltage operating circuits for the fuel andair blowermotors are electrically disassociated from the low voltage devices whichare adapted to control them, whereby the possibility of false operationor circuit feed-back" is avoided, and a positive and reliable operationof the stoker is assured.

An additional object is to provide an arrangement whereby the static airpressure in the air duct caused by the accumulation of fuel upon andwithin the fire bed, controls the operation of the fuel feed motor insuch a manner as to assure a predetermined fixed depth of fire bed,regardless of the varying the heating plant.

The foregoing and other objects and advantages .reside in certain novelfeatures of con- 30 structi'on, operation, arrangement and cooperation'of the various parts and elements of the system which will behereinafter more fully described and particularly pointed out in theappended claims, and of which a preferred embodiment is illustrated inthe accompanying sheet of I drawings forming a part thereof.

Referring to the drawing; a conventional type' air'conduit or duct Itextends from the retort 8 and is provided with a manually operateddamper control ll therein. A blower or fan I! operated by an electricmotor I3 ism-ranged to provide forced combustion air to the retort 8through the air duct ll.

demands for heat upon' ignated as A.

theretogby a suitable air passage or air pipe I 4 is a pressureresponsive device such as the static or b'ack pressure responsive switchgenerally des- This pressure switch may be of any well known type orconstruction and, for

example, comprises upper and lower walls l5 and,

ward through the top wall l5 of the chamber'is a stem or plunger l8.Plunger I8 is adapted to engaged the lower side of a lever l9 andactuate the same as the air pressure within the chamber I and below thediaphragm I1 is increased, and the diaphragm is pushed upward. Lever I9is adapted when operated to open a set of contacts 2| when the'static orback pressure reaches a predetermined pressure value. In order to adjustthe particular value at which the pressure responsive device actuatesthe contacts 2|, an adjusting means is provided which comprises acompression spring 22 bearing against the top of the lever H! at anydesired compression depending on the adjustment of an adjusting screw23.

A step-down transformer 24 is provided, having its primary windingconnected directly across a set of high voltage line conductors, 25 and26 for feeding current to the system. The conductor 25 is the groundside of the line while conductor 26 is the hot side. The low voltagerelay winding 21 which is adapted to close its contacts 28 whenenergized and complete a high voltage circuit to the blower motor l3. Aman- 'ually operated switch 29 is also connected in the A secondstep-down transformer provided which has its primary winding connectedacross the line conductors and 26 and one side of its secondary windingto the winding of of relay 42 to energize the same independent ofsecondary of transformer 24 is connected to a nace temperatures isincluded in the hot line' 26 in series with the contacts 28, or in anyother location where it can disable the. system when operated.

An interval timer is provided in the system such as the timer indicatedgenerally at B, comprising an electric synchronous motor 33 connectedacross the line conductors 25 and 26 for continuous operation. adaptedto control the operation of a pair of cams at predetermined periodicalintervals. One of the cams 34 is provided with an abutment arranged toactuate or close a pair of cam contacts 35 in parallel with the contacts32 of the room thermostat 3| so as to energize relay winding 21independent of the thermostat. The cam 34 is arranged to close thecontacts 35, preferably for a time interval of about three minutesduring each hour, the remaining 51 minutes of the rotation of thecampermitting the contacts.

The motor 33 is the cam contacts 31. The cam contacts 31 are adaptedwhen closed to energize the relay winding 42.

It will be noted that the operating circuits for the fuel motor 1 andthe blower motor l3 are all connected to the high voltage line circuit25 and 26 whereas the controlling devices such as the relays camcontacts, etc. for operating the motors are all connected in isolatedindependent low voltage circuits. This arrangement insures that therelatively heavy motor loads will not.

adversely affect the operation of the necessarily delicate associatedcontrolling devices. It will be seen that the arrangement of the systemprecludes the possibility of feed-back. and resultant false operation.The controlling contacts 28 and 44 break the hot line 26 for the motorsOperation In the operation of the system assume that in the ordinaryvsequence of events the interval timer rotates the cam 36 to a positionin which its abutment closes the contacts 31. This closure occurs, ashas been stated, for a time interval of one minute out of each sixminutes of operation. The closure of cam contacts 31 completes theenergizing circuit of relay 42 through the secondary of transformer 4|.Relay 42 upon operating closes a locking circuit for itself through itscontacts 43in series with the pressure switch contacts 2| andindependent of the .cam contacts 31. Relay 42 therefor remains held inits energized condition after thecam contacts 31 are opened or for anindefinite interval of time, bridging the interval between the time.

tor 1. It is seen that the relay 42 can be-initially. energized onlywhen the cam contacts 31 closev and may then thereafter be heldenergized through its locking contacts 43 after cam contacts 31 open.The relay is deenergized, how -fl ever, only when the pressure switchcontacts 2| open. From the foregoing it is seen that after an intervalof time which cannot exceed five minutes after the deenergization ofrelay 42 the fuel motor 1 is prepared for operation. This operation willoccur when the condition'of the fuel bed is such that additional fuel isrequired to be fed to the fire bed, as will be, pointed out.--

the room thermostat 3| is located, at a comfortl able value. mildweather when there is little call for heat, the room thermostat wouldoperate relatively infrequently. However, the interval timer 33,operating continuously, would function to maintain the fire alive. Thisis accomplished by the hour cam 34 which functions to close cam contact35 for a time period of approximately three minutes in each hour. Therelay winding 21 is thereby energized resulting in the closing of itscontacts 28. A high voltage circuit is thereby completed from the hotline conductor 26 through the blower motor I3 to the ground lineconductor 25. The blower l2 operates and forces air through the air ductIll into the retort 9 and through the fuel bed thereon. If there is asuflicient-amount of coal upon or within the retort, which may be thecase immediately after an operation of the fuel motor I, the backpressure within the air duct is equal to a certain amount due to theresistanceof the fuel bed. This pressure appears within the pressureresponsive device A and the dia- Under these conditions and during thecam contacts 35 are closed, the blower motor I3 would continue tooperate until the bed of fuel has been reduced or burned downto therequired depth and the static pressure reduced to permit the pressureswitch contacts 2| to close. If the room thermostat is still calling forheat at this time, the closing of pressure switch contacts 2| wouldallow relay 42 to operate (if it is in a position to do so) and feedfuel to the fire bed. This fuel feed would continue until suflicientresistance is again built up to cause opening of contacts 2|. In thismanner the fuel feed to the burner is automatically regulated to therate of burning of the fuel. The volume of air delivered to the burnercontrols the amount of fuel burned, and the operation of the pressureresponsive device A determines the amount of fuel fed to regulate thedepth of the fuel bed. By adjusting the screw 23 and thereby the forceof the spring 22 phragm I1 is forced upward to open contacts 2|. If,however, the fuel bed is not of the required depth, the restriction tothe passage of air through the fuel bed would be nominal and thepressure within the air duct l0 would not be sumcient to causedeflection of diaphragm H to.

The foregoing operations of the blower cannot, howeveLendu-re very longopen contacts 2 since the cam is effective each five minutes or may beeffective at the same timeas cam 34 to cause fuel to be fed to the firebed, as will be pointed out.

In the event that the relay 42 is locked enerphragm I'I.

gized at the time the cam contacts 35 are closed or the room thermostat3| is calling for heat,-

which will always be the case under normal operation, a circuit isclosed for the fuel motor I, through the relay contacts 44 and the relaycontacts 28, and the screw 6 feeds fuel to the fire bed at the same timethe blower I2 is forcing combustion air to the same. As this fuelcontinues to build up within the. retort 3-, the static or back pressurewithin the air duct I0 gradually increases due to the increase inresistance to the forced air caused by we accumulation of fuel. At thepredetermined pressure value to which the adjusting spring 22 has beenadjusted, the movement of the diaphragm |'I causes an open n of thecontacts 2|. This results in the deenergiza- .tion of relay 42 whichthereupon opens its contacts H and causes a stoppage of the fuel motorI. The blower, however, continues to run. At

contacts 43 the locking circuit for. relay 42 is opened and the relay 42can only energize again when the cam contacts 31 close in five minutes,

relay 21 to disconnect the high voltage operating circuit extendingthrough relay contacts 28.

From the foregoing it is seen that the interval timer sets up a startingand running circuit for the fuel motor whereas the pressure responsiveswitch A disrupts the running circuit for the 9 and flows through thetuyere passages, it-en-' counters more or less resistance, dependingupon the depth of the fuel bed through which the air must travel.Assume'now that as the system is operated, there is a thin fuel bed theburner. The resistance to air flow is then very low and the static orback pressure created in the air duct ID will be correspondingly low andtherefor insufficient to actuate the pressure dia- The contacts 2|therefor remain closed and the fuel motor continues to feed fuel. As thesupply of fuelto the burner increases, the resistance tothe airincreasesand eventually the static or back pressure will becomesuflicient to actuate the diaphragm I! and open contacts 2| causing thedeenergization of relay 42 and the stopping of stoker motor I.

The air duct damper Il may be set for any desired volume of air supplyfor the burner and if it is set wide open it permits maximum capacity.Under'these conditions the fuel'bed is set to be relatively thick at alltimes and the adjustment of the diaphragm pressure spring. 22 iscorrespondingly set so that the static or back pressure will,

operate the diaphragm only when the maximum amount of fuel has been fedto the fuel bed. A setting .for a relatively thinner fuel bed is madewhen the damper II is adjusted for a smaller amount of air.

It is usual in the operation of stokers' that occasionally the top ofthe burning fuel bed cokes over to form a substantially non-porouslayer. Sliculd this occur, obviously the resistancebto the air flowthrough the fuel bed would become quite high and the contacts 2| wouldbe opened regardless of the thickness of the fuel bed. I Therefore,under theseconditions, if the fuel b'ed were dependentupon the action ofthe contacts 2| alone, fuel feed would be prevented by this actionalthough fuel were actually required.

- To alleviate this difliculty the cam 36', by closing fuel motor when.the proper depth of fuel bed has been reached. '-Should the roomthermostat 3| continue to call for, heat for a longer time than thecontacts 31, causes fuel to be fed into th'e retort one'xninute out ofeverysix, regardless of other conditions, so long as the blower I3 isoperating. This action serves to break up the coked layer and providenormal air flow through the fuel bed.

The normally closed switch 23 in the circuit 'of relay 2! enables thestoker to be completely shut down as during the summer months or whenrepairs and inspections are necessary. The push button switch 45 enablesthe system to be tested for operation at any time without the necessityof waiting the five minutes required for the cam contacts 31 to close.

If desired, a single transformer may be used for supplying low voltagecurrent toxthe two reretort, fuel feeding means, air feeding means, a n

fuel feed electric motor forthe fuel feeding means and an air feedelectric motorfor the air feeding means, the combination of, a firstrelayarranged to control both motors, said relay when in a firstposition placing both the fuel feeding means and the air feeding meansout of operation and when in a second position placing the air feedingmeans in operation and permitting operation of the fuel-feeding means, alow voltage operating circuit for the first relay, a switching deviceresponsive to the demand for heat for opening and closing said lowvoltage operating circuit, a first time controlled switch for closingsaid operating circuit independently of said switching device atpredetermined intervals, a second relay arranged to control the fuelfeed motor for stopping and starting the fuel feeding means whilepermitting continuous operation of the air feeding means, said secondrelay having a main switch and a holding switch, the main switchcontrolling the fuel feed motor, a low voltage pull-in circuit for saidsecond. relay, a,

second time controlled switch arranged to be closed periodically atintervals more frequent than the first time controlled switch, saidsecond time controlled switch being interposed in said pull-in circuitfor causing said second relay to pull in at predetermined intervals, aholdingcircuit for maintaining said second relay pulled in independentlyof said pull-in circuit, said holding circuit extending through saidholding switch of the second relay, a pressure responsive switchresponsive to the pressure of the air sup- -plied to the retort andarranged to be closed when the air pressure is below a predeterminedvalue and to be open when the air pressure is above said value, saidpressure responsive switch being interposed in said holding circuit tomain tain said second relay pulled in until the air pressure rises abovesaid predetermined value, whereby said fuel feeding means is operated atpredetermined intervals of time for periods of variable duration, theduration of said periods varying with the thickness of the fuel bed.

2. In a control system for a stoker having a retort, fuel feeding means,air feeding means, a

fuel feed electric motor for the fuel feeding means and an airfeedelectric motor for the air feeding means, the combination of, afirst relay arranged to control both motors, said relay when in a firstposition placing both the fuel feeding means and the 'air feeding meansout of operation and when in a second position placing the air feedingmeans in operation and permitting operation of the fuel feeding means, alow voltage operating circuit for the first relay, a switchingdeviceresponsive to the demand for heat for opening and closing said lowvoltage operating circuit, a second relay arranged to control the fuelfeed motor for stopping and starting the fuel feeding means whilepermitting continuous operation of the air feeding means, said secondrelay having a main switch and a holding switch, the main switchcontrolling the fuel feed motor, a low voltage pull-in circuit for saidsecond relay, a time controlled switch arranged to be closedperiodically at intervals, said time controlled switch being interposedin said pull-in circuit for causing said second relay to pull in atpredetermined intervals, a holding circuit for maintaining said secondrelay pulled in independently of said pull-in circuit, said holdingcircuit extending through said holding switch of the second relay, apressure responsive switch responsive to the pressure'of the airsupplied to the retort and connected to said second relay, said pressureresponsive switch assuming a first position when the air pressure isbelow a predetermined value for maintaining the second relay pulled inthrough said holding switch, while assuming a second position in whichit causes deenergization of the second relay irrespective of saidholding switch when the air pressure rises abovea predetermined value,whereby the fuel feeding means is operated at'predetermined intervals oftime for periods of variable duration, the duration of said "periodsvarying with the thickness of the fuel bed.

V'IRGIL ROBINS, TATE.

